Same issue here. Upgraded from 8.1.3 to 9.0.1 08-30-2022 16:25:49.014 +0200 ERROR KVStoreBulletinBoardManager [166467 MainThread] - Failed to migrate KV Store storage engine to WiredTiger. This prevented running the latest version of KV Store. Migrate KV Store storage engine to WiredTiger and then upgrade KV Store. The Mmapv1 engine is deprecated. Managed to set engine to wiredTiger and status reports a OK, but message in webGui state Failed to upgrade KV Store to the latest version. KV Store is running an old version, service(40). Resolve upgrade errors and try to upgrade KV Store to the latest version again. when I gave command: splunk show kvstore-status --verbose I got listed following Info: serverVersion : 4.0.24 after executing migration withouth type of storage engine: splunk migrate migrate-kvstore I got upgraded version serverVersion : 4.2.17 Now Web GUI lost error and all works fine ! I used solution from the following post: https://community.splunk.com/t5/Installation/How-to-upgrade-Mongo-in-Splunk-9-0-0/m-p/603185#M11666
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